Steam methane reforming is a process that may be used to produce synthesis gas and hydrogen. A hydrocarbon and at least one of steam and carbon dioxide are reacted or reformed in the presence of a catalyst to produce hydrogen mixed with oxides of carbon, called synthesis gas or syngas. The reforming reaction is often performed within the radiant zone of a furnace heated by combustion of a fuel and an oxidant such as air.
Prior to entering the furnace the hydrocarbon is preheated, the steam, for example, is vaporized and superheated, the hydrocarbon and steam are combined into a common stream, and the resulting mixed feed is preheated against the flue gas to as high a temperature as possible to lower the firing requirements of the furnace and recover heat from the flue gas without causing the hydrocarbon to crack or precipitate solid carbon deposits. The desirably high temperature for the mixed feed to enter the furnace is normally in the range of 500° to 650° C., more often being about 650° C.
It is advantageous to recover heat from the resulting hot syngas and products of combustion or flue gas. Heat is normally recovered from the syngas in a first or syngas heat recovery section and from the flue gas in a second heat recovery section called a convective section.
Heat recovered from the syngas is utilized to preheat the hydrocarbon feedstock and to vaporize steam in what is called a process gas boiler.
The flue gas commonly performs diverse heating duties in the convective section. One duty is to vaporize and superheat steam. Another duty sometimes practiced is to preheat the air combusted in the furnace. An additional duty is to preheat the mixed feed of superheated steam and a preheated hydrocarbon feedstock to as high a temperature as possible without incurring carbon deposition.
Where efficient heat recovery is practiced, such as in a large steam methane reformer (“SMR”), the total amount of steam vaporized in the first and second heat recovery sections greatly exceeds the amount of steam required for the steam reforming process. The excess steam must be exported to another process unit in a chemical or power generation complex or refinery. Steam export typically reduces the net energy consumption of the SMR process by about 10%, which is substantial, but makes the SMR dependent on an external demand for the excess steam.
It is generally less expensive to build chemical plant equipment in a fabrication facility than in the field. SMR furnaces can be built in cylindrical form and assembled within a fabrication facility up to a very limited production capacity. For larger SMR furnace capacities it is necessary to build and assemble furnaces in the field. Although multiple modular furnaces may have lower installed costs than a single field assembled furnace, linking more than two furnaces to a common large flue gas heat recovery section is difficult. The difficulty in linking more than two furnaces to a common convective section is that the flow of fluids for the combination of heat duties of steam raising and superheating together with mixed feed preheating can become imbalanced between a single, common convective zone and three or more reforming furnaces. Multiple small convective sections have higher combined installed costs than a common, large convective section.
FIG. 1 illustrates an exemplary conventional bayonet reformer tube system 100. A bayonet reformer tube 101 is disposed within a furnace 102. The tube consists of an outer tube 103 and an inner tube 104. An annulus between the inner and outer tubes contains a reforming catalyst 105 (shown in FIG. 1 by cross-hatching). Gas enters the tube from an inlet header 106, flows downward through the annulus containing the catalyst, transfers to the inner tube 104 near a tip 107 of the tube, where the volumes of the annulus and inner tube are in communication with each other. The gas then flows upward from the tip 107 via the inner tube 104, and exits the tube to flow into an outlet header 108. Gas is heated and reformed in the annulus against combustion heating from the furnace and against the resulting heated syngas flowing through inner tube 104. Syngas flowing through inner tube 104 is cooled against the gas flowing through the annulus.
FIG. 2 illustrates a conventional SMR 200 and flow schematic. A hydrocarbon feedstock 281 enters SMR 200 via a line 202 and is conveyed to a heat exchanger 203 wherein the feedstock is heated against hot syngas. The preheated feedstock is conveyed by a line 204 to a desulfurization unit 205 wherein it is desulfurized and is then conveyed by a line 206 to a be mixed with superheated steam, together forming mixed feed.
Boiler feed water (“BFW”) 282 enters SMR 200 via a line 207 which conveys the BFW to a heat exchanger 208 wherein the BFW is heated against hot syngas. The preheated BFW is conveyed by a line 209 to a heat exchanger 210 wherein it is vaporized against flue gas. The resulting steam is then conveyed by a line 211 to a heat exchanger 212 wherein it is superheated against flue gas. A portion of the superheated steam 283 in excess of the reformer requirements is exported from SMR 100 via a line 213. The balance of the superheated steam is conveyed via a line 214 wherein it is mixed with the feedstock, forming mixed feed, and to a heat exchanger 215, wherein the mixed feed is preheated to the inlet temperature of the reformer furnace. The preheated mixed feed is conveyed via a line 216 to a reformer tube 217 within a reformer furnace 218, wherein the mixed feed is heated and reformed against heat from the furnace. The resulting reformed hot syngas exits the furnace and is conveyed via a line 219 to heat exchanger 208 (which may be a process gas boiler, for example) wherein it is cooled against BFW, is conveyed via a line 220 to a water gas shift reactor 221 wherein some of the steam and carbon dioxide contained in the syngas react to form additional hydrogen and carbon dioxide, is conveyed via a line 222 to heat exchanger 203 wherein it is cooled against BFW, is conveyed via a line 223 to a fin fan (or heat exchanger) 224 wherein it is cooled against ambient air, is conveyed via a line 225 to a water knockout unit 226 wherein condensed steam is separated from the syngas, and is conveyed via a line 227 to a pressure swing adsorption or PSA unit 228 wherein most of the hydrogen is separated from the remainder of the syngas. Hydrogen 287 exits the PSA unit 228 as a hydrogen product via a line 229, and the remainder of the syngas exits the PSA unit 228 as a tail gas via a line 230 and is conveyed to the furnace burners wherein the tail gas is combusted.
Combustion air 284 enters SMR 200 and is conveyed via a line 231 to a heat exchanger 232 wherein it is preheated against flue gas. The preheated combustion air is conveyed via a line 233 to the furnace burners wherein the air is combusted with tail gas and supplemental fuel to heat the furnace. Supplemental fuel 286 enters SMR 200 and is conveyed via a line 234 to the furnace burners wherein it is combusted to heat the furnace. Combustion products exit the furnace as flue gas 285 via a convection section 236 and are progressively cooled as they sequentially pass through heat exchangers 215, 212, 210, and 232. The flue gas 285 then exits SMR 200.